gistfile1.txt

#include <stdio.h>

void foo(int* x) {
  printf("%d\n", x[3]);
}

int main() {
  int arr[3];
  int overflow = 1000;
  arr[0] = 10;
  arr[1] = 20;
  arr[2] = 30;
  foo(arr);
  return 0;
}

Analyzing the results of gcc -O0 -masm=intel gistfile1.c should pretty much tell you what's going on, if you use it diagram out the stack frame of main.

On my Ubuntu desktop, int overflow = 1000; translates to mov DWORD PTR [rbp-36], 1000, and arr[2] = 30 translates into mov DWORD PTR [rbp-24], 30. arr[3] would be the contents of [rbp-20]. As Franklin points out, the allocation of stack ordering is not defined.